The present invention relates to an Optical Metro-Access Transmission Based on Wavelength Division Multiplexed Orthogonal Frequency Division Multiple Access Passive Optical Network (WDM-OFDMA-PON).
Next-generation passive optical networks (PON), with 40+ Gb/s target data rates and aggressive power budgets that can accommodate both increased PON reach and higher optical network unit (ONU) split ratios, have become a highly prominent topic in optical access research and commercialization activities. Moreover, it is important to ensure PON cost-efficiency, such it is highly desirable to maximally reuse deployed legacy fiber with last-mile passive optical splitters, which account for 70-80% of PON investment costs. Finally, to lower PON operational costs and power consumption, a dramatic reduction in the number of central office (CO) sites in the access network is needed. Under such a scenario, many COs servicing shorter-reach, lower split ratio PONs would be merged into a single CO that would service a long-reach (100+ m), large split ratio (1000+) PON, with up to 1 Gb/s peak symmetric downstream/upstream data rates per user. Such a high-performance, long-reach PON would essentially converge the hereto separate metro and access optical networks into a single high-speed platform.
A promising approach for such metro-access convergence with a passive last-mile split is a hybrid network based on dense wavelength division multiplexing (DWDM) combined with a colorless last-mile multiple access technology, such as time division multiple access (TDMA), for example. However, in TDMA-based last-mile approaches, the per-wavelength (i.e. per λ) speed is limited to 10 Gb/s/λ, and the aggregate speed to 32λ×10 Gb/s/λ=320 Gb/s, due to the requirement for high-speed upstream burst-mode operation. Moreover, both inline optical dispersion compensation and time-domain electronic equalization were required to mitigate the chromatic dispersion (CD)-induced power budget penalty. With experimentally demonstrated per-wavelength data rates of 40-108 Gb/s/λ, digital signal processing (DSP)-based Orthogonal Frequency Division Multiple Access (OFDMA)-PON has emerged as an attractive candidate for next-generation optical access. Next to record per-channel speeds, OFDMA-PON advantages include a passive last-mile split, very high CD tolerance, as well as burst-mode-free upstream operation via dynamic frequency-domain bandwidth allocation. From this perspective, OFDMA-PON is a highly-promising technology to realize high-speed network convergence.
FIG. 1 shows a conventional WDM-converged metro/access network with a WDM-PON last mile. At the central office optical line terminal (CO-OLT), the output signals of a 32λ×1.25 Gb/s/λ transmitter array (100) operating on DS wavelengths λ1, DS to λ32, DS is combined with an arrayed waveguide grating (AWG) in (200) and aligned onto a 50 GHz ITU-T grid for downstream transmission over standard single mode fiber (SSMF) in (203). A practical choice of the ITU-T grid can be the 1529-1541.6 nm portion of the C-band, for example. The modulated dense WDM (DWDM) signal is transmitted over the metro network (203), which consists of D km of standard single mode fiber (SSMF). In the access network, a second AWG (301) is used to de-multiplex the aggregate signal into 32 separate 1.25 Gb/s signals, which pass through a diplexer (304), d km of SSMF (305), and a second diplexer (401) in point-to-point fashion to reach one of the 32 optical network units (ONUs). At each ONU, the DS signal on λ1, DS is received in the ONU receiver (402). For upstream 32λ×1.25 Gb/s/λ transmission, a 1.25 Gb/s ONU transmitter (500) is used, which is made up of a reflective electroabsorption modulator (R-EAM) and semiconductor optical amplifier (SOA), known in prior art. The ONU transmitter (500) in each of the 32 ONUs locks onto and modulates a dedicated upstream wavelength, λ1, US to λ32, US. The modulated upstream signal λi,US,mod is then routed through (401), (305), and (304), in that order, to the second AWG (600), where the aggregate upstream signal is launched over the metro network SSMF (602) to the CO-OLT. At the CO-OLT, the incoming wavelengths are de-multiplexed in an AWG (701) and processed individually by the 1.25 Gb/s CO-OLT receiver array (800). The advantages of the conventional WDM-PON approach of FIG. 1 includes the use of cost-efficient 1.25 Gb/s optical components, the low insertion loss of the AWG compared to the last-mile passive optical splitter, and physical layer security. However, the approach of FIG. 1 would also require fundamental network alteration (i.e. the removal of all deployed passive splitters and replacement with AWGs), which would entail enormous additional cost to network operators. Moreover, the per-user peak bandwidth is limited to 1.25 Gb/s, without the ability to statistically multiplex multi-user traffic. Finally, the speed of this architecture is limited to 32λ×1.25 Gb/s/λ=40 Gb/s, with only 32 users (ONUs) accommodated per fiber.
FIG. 2 shows a variation of the WDM-PON of FIG. 1, which features similar operation as the conventional WDM-PON of FIG. 1, except that it removes the requirement for an AWG in the last mile network, such that the deployed passive optical splitter (400) can be re-used. This is achieved in FIG. 2 by equipping the CO-OLT with an array of broadband transmitters (100) and receivers (800), and the ONUs with an optical coherent receiver (402) featuring a tunable laser that can lock onto the upstream wavelength λi, US based on the downstream wavelength reference λi, DS that is pre-assigned to it at the CO-OLT. This tuning and locking feature is known from prior art. In this way, colorless WDM operation is achieved without the need for reflective ONU-side optical devices or a last-mile AWG split, and the coherent receiver (402) can enable receiver 3+ dB sensitivity gains compared to equivalent non-coherent detection. However, due to the use of 1 GHz optical components for cost-efficiency, the peak data rate is still limited as above without the possibility for statistical multiplexing of user bandwidth. Moreover, since low-cost tunable lasers are not yet commercially available, the cost-efficiency of coherent ONU receiver (402) is questionable.
FIG. 3 shows a conventional WDM-converged metro/access network with a TDMA-PON last mile. At the CO-OLT, the output signals of a 32λ×10 Gb/s/λ transmitter array (100) operating on DS wavelengths λ1, DS to λ32, DS is combined with an AWG (200) and aligned onto a 50 GHz ITU-T grid. Next, dispersion compensating fiber (DCF) in (202) is used to mitigate the chromatic dispersion effect, which is followed by downstream SSMF transmission (203). At the local exchange (LE), optical amplification (OA) and DWDM de-multiplexing are performed in (204) and (301), respectively, with each 10 Gb/s/λ downstream signal distributed to N users by a diplexer (304), SSMF (305), and a passive optical splitter (400). After a second diplexer (401), the downstream signal is received in (402) by a 10 GHz photodiode. Due to TDMA, bandwidth usage can be apportioned in a time-domain round-robin fashion among multiple ONUs, such that during each time slot, only one ONU can transmit or receive. Consequently, the peak data rate of 10 Gb/s is statistically shared between N users, where N can vary from 32 to 256. For upstream transmission, ONU-side R-EAM-SOAs (described in prior art) in the ONU transmitter (500) are used to modulate upstream data at each ONU, such that a 10 Gb/s aggregate TDMA upstream signal from N ONUs, λi,US,mod, occupies one of 32 upstream wavelengths. Following (401), (400), (305), and (304), in that order, the upstream signals from multiple wavelengths are aggregated by the upstream LE AWG (600), optically amplified (601), and launched over the metro SSMF (602). The upstream wavelengths are de-multiplexed at the CO-OLT AWG (701), and the 10 Gb/s signal from each λi, US is received by a high-speed 10 Gb/s burst-mode receiver (800). The main advantages of the WDM-TDMA-PON approach are the re-use of last-mile passive splitter (400), and the ability to accommodate up to 32×256=8192 users (ONUs) per fiber. However, in TDMA-based last-mile approaches, the per-wavelength (i.e. per λ) speed is limited to 10 Gb/s/λ, and the aggregate speed to 32λ×10 Gb/s/λ=320 Gb/s, due to the requirement for 10 Gb/s upstream burst-mode operation (800), which cannot readily scale to a 40 Gb/s/λ line rate. Moreover, due to the 10 Gb/s line rate, both inline optical dispersion compensation (202) and time-domain electronic equalization following (800) are required to mitigate the chromatic dispersion power budget penalty.